1. Field of the Invention
The present invention relates to a filter and, more particularly, to an electromagnetic interference filter.
2. Description of the Related Art
There are problems of interference in circuits of the electronic devices that share one power supply device, and the most is electromagnetic interferences (EMI). A power supply unit is essential to almost every electric appliance and always become a main source of electromagnetic interferences. For this reason, most power supply devices are provided with an electromagnetic interference filter (EMI filter).
FIGS. 1A & 1B are the lateral view and the top view of a conventional EMI filter 1 respectively. Referring to FIG. 1B, the capacitor 11 of the EMI filter 1 consists of two ceramic capacitors (also called the Y capacitors) 111 and a metallic film capacitance (also called the X capacitor) 112. FIG. 1C shows the assembly procedure of the capacitor 11. Since the ceramic capacitors 111 and the metallic film capacitance 112 included in the capacitor 11 are standard products, the cost of the insulating materials 1111 used to cover the ceramic capacitor 111 and the insulating materials 113 used to cover the metallic film capacitance 112 in the manufacturing process can be greatly risen. Besides, the illustrated method of assembling the ceramic capacitors 111 and metallic film capacitance 112 always increases the size of the capacitor 11, and thus increases the size of the entire EMI filter 1. Although, as shown in FIG. 1D, two metallic film capacitances 112′ and 112″ can be combined to form the above-mentioned capacitor 11, the circuit of this capacitor 11 cannot be separated in application.
Thus, either the compounded capacitor 11 made up of a metallic film capacitance and a ceramic capacitor or the compounded capacitor 11′ made up of two metallic film capacitances has limitations in the EMI filter application.
Referencing back to FIGS. 1A and 1B, there are only three electric circuit contacts required for the capacitor 11, i.e. a, b, and c as shown. Therefore, the repeat procedure of arranging the four wires of the ceramic capacitor 111 will be needed and the cost for the operation will be raised as well. In addition, the entire EMI filter 1 further requires additional non-electronic components such as printed circuit board 13 to establish electric circuit contacts, and electrodes 14a and 14b such as PVC line or metal terminal to create the input/output terminals. Therefore, from the entire point of view, the conventional EMI filter has high manufacturing cost and low economic benefits, and does not meet the production need.